You may be able to use the free Veeam Business View to help here. BV creates Categories and groups for your VMs. You can then place your VMs in these groups automatically (like Outlook email rules). Placement of VMs in groups auto creates custom attributes. VMs can be placed manually too, of course.
You can install BV, connect vCenter to it, and then it will ask you what to do with your existing custom attributes. So, you could create new categories and groups based on your existing ones. or you can manually import and, in the future export, custom attributes. Business View also provides nice web dashboards to help you report on your groups and even provide "show back info".
A restored VM would (ideally) automatically fall into it's appropriate cats and groups thus restoring the custom attributes. Worst case would be you manually place the restored VM in the right Cat/Group.
Check out Business View! It's free! http://www.veeam.com/vmware-business-view.html
It's primarily used to enhance Veeam Monitor and Veeam Reporter, but it seems like their is some value here for your backup scenario.